The H5N8 strain of bird flu has been confirmed in nine mute swans but testing stops once the presence of the disease has been established so it is not known how many have been struck by the disease. Usually 30-40 swans would be expected to die over a typical winter.

Measures are being taken to stop the disease spreading to poultry flocks but the 1,000 free-flying swans at Abbotsbury are wild and it is not possible to completely confine them.

Avian flu has been detected in poultry flocks from Lincolnshire to south-west Wales and as far north as Yorkshire as well as in wild birds across the country.

Staff at Abbotsbury swannery, which is thought to have been established by Benedictine monks in the 11th century, spotted mute swans looking lethargic and then dying shortly before Christmas.

It sent the bodies for testing and the H5N8 strain was confirmed in nine birds.

John Houston, the Abbotsbury Tourism general manager, said the welfare of the birds and of swannery staff was the priority.

“Cases of avian flu have been reported in wild birds in Europe and in a number of locations across the UK. The current H5N8 strain of bird flu is of very low risk to public health and has never transferred to humans, but the wellbeing of the swans and other birds that visit the swannery is paramount,” he said.

“The swannery is closed to the public for the winter and is not due to reopen until March. Current access to the site is therefore restricted to staff members only. We are closely monitoring the health of the swans and our staff are taking all necessary precautionary measures as advised.”